1. Field of the Invention
The present invention relates to an SOI (silicon on insulator) structure wafer obtained by a method wherein an ion-implanted wafer is bonded to another wafer and a portion of the ion-implanted wafer is delaminated (separated) to provide an SOI wafer (called a smart-cut method). More particularly, the present invention relates to a method of fabricating an SOI wafer having an active SOI layer which is excellent in thickness uniformity, and an SOI wafer having an active SOI layer which is excellent in thickness uniformity.
2. Description of the Related Art
Recently, a method of fabricating an SOI wafer in which an ion-implanted wafer is bonded to another wafer and a portion of the ion-implanted wafer is delaminated to provide an SOI wafer (hereinafter, occasionally referred to as an ion implantation and delamination method) has attracted attention. In this method, as shown in FIGS. 3(a) to (h), an oxide film is formed on the surface of at least one of two silicon wafers (FIG. 3(b)), which are a bond wafer 2 to form an SOI layer and a base wafer 1 to be a supporting substrate (FIG. 3(a)); hydrogen ions or rare gas ions are implanted into the bond wafer 2 in order to form a fine bubble layer (enclosed layer) 4 within the bond wafer (FIG. 3(c)); the ion-implanted wafer is superposed on the base wafer 1 such that the ion-implanted surface comes into close contact with the surface of the base wafer 1 via the oxide film (FIG. 3(d)); heat treatment is then performed to delaminate a portion of the bond wafer 2 as a thin film with using the fine bubble layer as a delaminating plane (FIG. 3(e)); heat treatment is further performed to firmly bond the wafers to each other (FIG. 3(f)), which are then subjected to mirror polishing (FIG. 3(g)) to thereby obtain an SOI wafer (FIG. 3(h)) (See Japanese Patent Application Laid-Open (kokai) No. 5-211128).
The above-mentioned method can be roughly classified into two methods depending on the wafer on which an oxide film is formed. In one of them, Method A, the oxide film is formed on the base wafer 1 as shown in FIG. 3(A), and in the other of them, Method B, it is formed on the bond wafer 2 as shown in FIG. 3(B). Method B wherein the oxide film is formed on the bond wafer in advance has been performed mainly.
Because, the depth of ion implantation varies widely due to a channeling effect, when the oxide film is not formed on the bond wafer in which ions are to be implanted, and as a result, thickness uniformity of the SOI layer may be lowered.
"The channeling effect" means herein a phenomenon wherein ions implanted parallel to the crystal axis of the crystalline material pass zigzag between the atoms of the crystal. In that case, the deviation in the ion implantation depth is larger, compared to the case that the ions are implanted nonparallel to the crystal axis. The phenomenon is apt to occur particularly in a silicon wafer, since the surface thereof is processed in a certain orientation (for example, &lt;100&gt;). Accordingly, it is preferable to form the oxide film on the wafer to suppress the channeling effect in that case.
Another reason why the oxide film should be formed on the bond wafer is that the oxide film previously formed on the bond wafer may suppress diffusion of impurities incorporated in the bonded surface (boron in atmosphere or metal impurities due to ion implantation) into the active layer (SOI layer), and therefore, degradation of crystallinity of the SOI layer and electronic characteristics can be prevented.
When the channeling effect is not caused, deviation (standard deviation) .sigma. of the ion implantation depth in the ion implantation and delamination method can be 0.4 nm. Namely, 3.sigma.=1.2 nm can be achieved, so that almost all of ions can be implanted at an intended depth .+-.1.2 nm. Accordingly, it is inferred that a super thin SOI wafer having good thickness uniformity of an intended thickness .+-.1.5 nm or less can be obtained according to the ion implantation and delamination method.
However, when the oxide film is formed on the bond wafer in which ions are to be implanted because of the above-mentioned reasons, there is a deviation also in the thickness of the formed oxide film, and the implantation depth of ions implanted in the silicon through the oxide film is also affected thereby.
For example, when a thickness of a buried oxide layer of an SOI wafer needs to be 400 nm, a standard deviation .sigma. in the thickness of the oxide layer will be 2.0 nm at the smallest under oxidizing condition in general mass-production. Even if the oxidizing condition is precisely controlled without considering productivity, .sigma. is around 1.0 nm at the smallest. Accordingly, a thickness uniformity of an SOI layer of a conventional SOI wafer produced with forming an oxide film on a bond wafer is around an intended thickness .+-.3 nm at the smallest.